Warning:Instantiation depth might indicate recursion in ModelSim

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I want to implement HA (half Adder), and I have a problem. When I click simulate, it displays this warning:

Warning: Instantiation depth of '/TM_HA/HA/HA/HA .....This might indicate a recursive instantiation.

I don't know what's going on.

HA.v

`timescale 1ns/1ns
module HA(a,b,c,sum,cout);
 input a,b,c;
 output sum,cout;

 wire e1,e2,e3;

 xor(e1,a,b);
 and(e2,a,b);
 and(e3,e1,c);

 xor(sum,e1,c);
 or(cout,e3,e2);

endmodule

TM_HA.v

`timescale 1ns/1ns
module TM_HA;
  reg A,B,Cin;
  wire Sum,Cout;

TM_HA HA(.a(A), .b(B), .c(Cin), .sum(Sum), .cout(Cout));

parameter t = 200;
initial
  begin
  #(2*t)
  A = 1'b0;
  B = 1'b0;
  Cin = 1'b0;

  #t
  A = 1'b0;
  B = 1'b0;
  Cin = 1'b1;
  ....
  #t
  $stop;
end
endmodule
verilog
2 Answers

As mentioned in the warning ModelSim is giving you: you are performing a recursive instantiation. Lets have a look at the first couple of lines of your TM_HA module:

module TM_HA;
  reg A,B,Cin;
  wire Sum,Cout;

TM_HA HA(.a(A), .b(B), .c(Cin), .sum(Sum), .cout(Cout));

Within the module TM_HA, you are creating an instance called HAof the parent module TM_HA. This is a recursive call, since that instance will again create another instance of TM_HA.

To actually instantiate your HA module, you should do (something like) this:

HA HA_inst (.a(A), .b(B), .c(Cin), .sum(Sum), .cout(Cout));
user_79493780

As you warning says, you have a recursive instantiation. This line 

TM_HA HA(.a(A), .b(B), .c(Cin), .sum(Sum), .cout(Cout));

inside module TM_HA means

instantiate module TM_HA and give it the instance name HA.

As that line is inside module TM_HA, you are instantiating module TM_HA inside itself - a recursive instantiation. At some point, between zero and infinity levels of hierarchy, you simulator gives up.

user_59000110
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